228 Mr. H. F. Newall. 



In estimating extensions in the unpolarised images no account was 

 taken of light, which it seemed fair to attribute to diffusion from 

 clouds. Extension was gauged along streamers, not in structureless 

 luminosity. In many cases bands can be traced in the polarised 

 images further than streamers in the unpolarised. 



Many years ago Sir G. G. Stokes pointed out that the Savart 

 polariscope might be used for the detection of faint extensions of the 

 corona, and it may be that these photographs give evidence of the 

 value of that suggestion. 



Across the dark moon very faint bands are detectable only in the 

 17-second exposure. I am not yet prepared to say how far the 

 discussion of the results may lead me to attribute the extension of 

 the bands to the diffusion of polarised light which emanates from the 

 corona by terrestrial clouds, but it is clear that the resultant polarisa- 

 tion of such light must be, so far as the Savart bands are concerned, in a 

 plane parallel to the equatorial extensions. 



The value of the photographs, as a series, with related exposures is 

 much interfered with by the variation in the effect produced by cloud, 

 and in consequence it will hardly be possible to utilise them for finding 

 at what distance from the sun there is the maximum relative polarisa- 

 tion, even were the impressed extensions great enough. But as each 

 plate had two exposures made on it simultaneously, one of the polarised 

 image and one of the unpolarised, it is anticipated that they will serve 

 well for the determination of the amount of polarisation in many 

 regions of the corona. 



The accompanying figure (fig. 1) indicates the contours of equal 

 brightness in the two images, polarised (dotted line) and unpolarised 

 (unbroken line) obtained in the second (the longest) exposure. It is 

 reduced from a drawing made on a screen on which the two images 

 were successively thrown by means of a lantern. The most marked 

 streamers in the corona are indicated as well as the contours. No 

 great precision has been aimed at in the diagram, its object being 

 merely to give an idea of the sort of material obtained for the 

 investigation. The phenomena are somewhat complicated; in con- 

 sequence of the radial polarisation of the corona, the bands in the 

 quadrants through which the central band passes form a system with 

 white centre, those in the other quadrants form a system with dark 

 centre ; the two systems meet along lines passing through the centre 

 and inclined at 45 to the central band. Hence some care must be 

 exercised in interpreting the appearances presented. 



14. Atmospheric Polarisation. 



The Savart polariscopes A, B, and C were fixed on the framework of 

 the hut in such position that the sky in the neighbourhood of the 



